The present invention relates to instruments used for analyzing the frequency content of electromagnetic signals, and more particularly to an improvement which can record indications of transient responses resulting from short duration single events in acousto-optic devices. The predominant current usage of the present invention is in the analysis of short pulse duration high bandwidth microwave frequency events, particularly in experiments where it is desirable to produce a record of relative intensities of event power content at various transient frequencies.
The properties of acousto-optic crystals have been previously employed in spectrum analysis devices in several different ways. U.S. Pat. No. 4,633,170 issued to Burns et al. teaches a method for improving the resolution of such a device. A common configuration of an acousto-optic crystal is a Bragg cell. A good general description of Bragg cells and their properties is contained within the Burns patent. In brief, a Bragg cell is a crystalline block with a piezoelectric transducer bonded to its side, such that when the transducer is excited with an electrical signal, a traveling acoustic wave is set up in the crystal. A light beam (almost always a laser beam) is passed through the block from end to end. The acoustic wave created by the electrical signal causes slight changes in the refractive index of the cell material resulting in a portion of the input light beam being deflected. When the Bragg cell is properly tuned, the amplitude of the deflected beam is proportional to the amplitude of the acoustic input, and the deflection angle is proportional to the frequency of the acoustic input. An additional characteristic of the Bragg cell is that the modulation on a signal is preserved. That is, if the input signal contains a number of different frequency components, each component will be deflected according to its frequency (within certain limits, as discussed herein), and the magnitude of each reflected portion will correspond to the relative power of that frequency present in the input signal.
The limiting factors of the usefulness of the Bragg cell in spectrum analysis applications have been mostly due to limitations in the apparatus for receiving and analyzing the diffracted light beams. These limitations have been manifested as limited dynamic range and limited frequency range. Another limitation has been the difficulty of correlating Bragg cell output to temporal changes in transient content over the duration of an event. For example, the most common method for receiving the light beams has been to arrange an array of photosensors, usually photodiodes, such that a the amount of defection of a beam can be ascertained according to which of the array is stimulated, and the magnitude of each deflected portion is then judged to be proportional to the magnitude of the electrical signal resulting from the light incident to each respective photosensor. However, using this method, the dynamic range is limited by the narrow range between the background electrical signal and the saturation point of such photosensors. Further, both the resolution and the range of such an instrument is limited by the number and placement of the photosensors. U.S. Pat. No. 4,722,596, issued to Labrum et al., teaches a method to extend the dynamic range of this type spectrum analyzer by effectively dynamically compressing each portion of the signal at its limit. U.S. Pat. No. 4,636,718, also issued to Labrum et al., teaches a method for extending the frequency range and/or resolution by time sharing the photosensor array among different portions of the frequency spectrum. These methods are very useful for general purpose type spectrum analyzers. However, they still do not provide nearly sufficient frequency range to analyze the entire content of many short duration high energy events. Nor do they allow a sufficiently quick recording time to accurately record such events. Nor do they provide a sufficiently detailed recording of the relative magnitudes of each component frequency to allow a careful and detailed scientific analysis. Of course, prior art methods can make a temporal record giving indications of changes of readings over time by the simple expedient of taking a series of readings as rapidly as the technology will allow. But given the need for time sharing, data recording time, and other factors which slow the process, a great deal of resolution over time is lost by this technique. In fact, changes which occur over extremely short periods of time may be lost entirely from the record.
Therefore, there is clearly a need for a method to allow for the capturing of the output of a Bragg cell which will produce a broad spectrum recording showing all the nuances and fine detail of the Bragg cell deflected light beams and, further, which will create this recording even when an event duration is very brief. Moreover, changes in the Bragg cell output should be recorded continuously, rather than intermittently, in order to preserve an accurate record of such changes over time. To the inventors' knowledge, no prior art method has provided this combination of broad frequency range, acceptable resolution, and quick recording time. Nor has any prior art method provided a means for continuously recording Bragg cell output over the duration of an event.